CUT_PHONE_LINE_ALARM_1

Published:2009/7/12 22:31:00 Author:May | From:SeekIC

Detecting a cut phone line can be an important function of an electronic security system. UnforLunately, detecting a cut phone line isn't easy because the voltages on a normal phone line vary so rnuch. The voltage is typically 48 Vdc when the telephone is on the hook, 2 to 15 Vdc during a conversation, 90 Vac during ringing, and 200 Vdc when the telephone company is testing the line. Brief moments of 0 V are common; what you really want to detect is a voltage that goes to zero and stays there. A second restriction is that the cut phone line detector cannot draw any appreciable current. Its impedance has to be higher than 50 MΩ or the telephone company will think it's a leaky cable, Components R1, R2, R3, and C1 smooth out momentary variations in voltage so that the alarm doesn't trigger every time the telephone rings. If the voltage stays at zero for 30 s or more, the alarm should trigger. The load can be a piezo buzzer, an optocoupler, or a small relay.Because of the tiny currents it rnust detect, this circuit should be powered by its own 9-V battery, with no direct connection to the rest of the burglar alarm. Otherwise, a slight difference in ground potentials might cause the circuit to perform somewhat erratically.



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